Server stops responding after a while-Help!

I am not sure where to start to track down this problem.
After a while (different each time) server stops responding.
I am unable to ping or shell into box to see what is up.
I hard reset box and all works ok then a few days or few hours later it stops responding again!! What logs should I look at or how should I track this to correct?
Please advise.
thanks in advance.
ubuntu perfect server 10.10 ispconfig setup
installed about 1 month ago...

log from when it died to after restart
Muni didnt really show anything just that it went to sleep and stopped too.
Could this be a power saving issue ? how to turn that off?
Thanks.. ps doesnt appear to be using swap at all